Tanacetum Vs. Jacobaea

Day 352: When people hear the word "tansy," they may be referring to plants of either genus, Tanacetum or Jacobaea. Both are toxic to livestock and contain volatile oils which may cause contact dermatitis in sensitive individuals, and both are considered invasive. The plants are easy to tell apart, especially when in bloom. Tanacetum vulgare (Common Tansy, top) has no ray flowers; Jacobaea vulgaris (aka Senecio jacobaea, Tansy Ragwort, bottom) does. The leaves are also distinctly different, although those of Tansy Ragwort can be highly variable, especially when young. The foliage of Common Tansy is fern-like; that of Tansy Ragwort is similar to that of radishes left too long in the garden, and forms a rosette. In Washington, the introduction of Cinnabar Moth (Tyria jacobaea) as a biological control has been fairly successful in reducing the occurrence of Tansy Ragwort, the caterpillars' primary food source. Once they have decimated an infestation of Ragwort, the caterpillars cease reproducing and therefore cannot become a secondary concern.